1. Field of the Invention
This invention relates generally to apparatus for processing Loran C signals and more particularly to apparatus for processing a received Loran C signal and compensating for envelope cycle discrepancy.
2. Description of the Prior Art
Loran C is a radio navigation system employed in the low frequency portion of the radio spectrum at a carrier frequency of 100 kilohertz. A typical Loran C chain of transmission stations includes a master station and at least two secondary stations each of which transmits a plurality of radio frequency pulses at pre-selected times relative to each other. The Loran C receiver operates by measuring the difference in time of arrival of the RF pulses transmitted by the transmitting stations. These time differences (TD) are the time measured between the arrival of the pulses from the master transmitter in each of the other secondary transmitters in the group. Each measured TD is a function of the receiver's distance from the master and the secondaries. To determine the location of the Loran C receiver, the TD between time of arrival of the first secondary station pulses and the master station pulses is determined and employed to plot a hyperbolic line of position on an appropriate map. The TD between the time of arrival of a second secondary station pulses and the master station pulses is determined and used to plot a second hyperbolic position intersecting with the above first mentioned line of position thus yielding the location of the Loran C receiver.
The signal transmitted from each station is a series of pulses of RF carrier at 100 kilohertz with a prescribed envelope, as shown in FIG. 1. The receiver locates and tracks pulses from each station by phase locking to one of the pulse's positive-going zero crossings, normally the third positive-going zero crossing designated as the pulse tracking reference (PTR). The TD's are typically obtained by gating on a counter at a time coincident with the arrival of the master station's tracked zero crossing and counting the number of time increments that occur until the counter is gated off at the arrival of the tracked zero crossing of the secondary. This method of measuring TD's requires that the same zero crossing is tracked on each station's pulses. If this were not true, the TD's measured would vary by increments of ten micro-seconds (one carrier cycle) each time it was necessary to acquire and track a station. The ambiguity would result in location errors of many miles, thus, the convention has been adapted to use the third positive-going zero crossing of the pulse as the PTR.
In normal operation, the Loran C receiver starts in a "search" mode, searching for and phase-locking to a zero crossing of the pulses transmitted by the master and the secondary stations. The Loran C receiver then enters the "coarse settle" mode, wherein a means is employed to jump the phase tracking point up toward the front of the pulse, such that it is known that the pulse tracking point is near the desired PTR. At this point, the receiver enters the "fine settle" mode, wherein the phase tracking point is jumped to the exact PTR. When fine settle has been completed on the master and at least two secondaries, TD measurement operations are started resulting in the output of location information. The critical operation is the "fine settle" mode. An error in tracking cycle selection in the fine settle mode can result in subsequent location errors of one or more miles.
Most Loran C receivers operate using pulses which have been hard limited to simplify the RF design and facilitate the interfacing of signal path through pulse sampling circuitry to digital circuitry. However, since the pulse envelope contains the information necessary to make a fine settle decision in the carrier cycle peak ratios, this information is lost in the hard limiting process. The first few cycles of a received Loran C pulse are shown in FIG. 2A, and the hard limited wave form as seen at the pulse sampling circuitry is shown in FIG. 2B. The limited waveform is binary in form and contains sufficient information for the receiver to carry out the search, coarse settle, and TD tracking functions. During the fine settle process, "envelope deriver" circuitry is switched into the RF signal path before the hard limiter for some of the received pulses. This circuitry adds a five microsecond delay and amplified version of the received pulse to the original pulse, resulting in a new pulse, (as shown in FIG. 3A) which contains a phase reversal at the PTR. This new waveform is hard limited producing a waveform such as that shown in FIG. 3B. The PTR can thus be determined by sampling the hard limited envelope derived pulses at 7.5 microseconds ahead and 2.5 microseconds behind the positive zero crossing times of the normal pulse near the one being tracked as a result of the coarse settle operation. When a change in state of the binary wave form is found along with a set of these leading (L) and trailing (T) samples, the PTR is the zero crossing point that lies between them.
In actual operation in a noise filled environment, statistical averaging of samples from many pulses is used for the fine settle operation. For example, five sampling strobes (see FS1 to FS5 in FIG. 3B) may be used on each pulse. They take samples of the derived pulse 17.5 and 7.5 microseconds ahead of and 2.5, 12.5, and 22.5 microseconds after the phase tracked zero crossing which was reached at the end of the coarse settle mode. The samples from each pulse are processed to increment or decrement the designated accumulator in a group of five corresponding to the samples FS1 through FS5. After samples from many pulses have been accumulated, a fine settle operation is started to examine each adjacent pair of sample accumulators as a potential leading or trailing phase reversal pair, starting with FS1 and FS2. When an adjacent pair of accumulators is found such that the leading 1 (designated L) is positive and greater than some predetermined positive fine settle threshold, and the trailing one (designated T) is negative and less than some predetermined negative fine settle threshold, the PTR is considered found and the phase tracking point is locked to the positive zero crossing that lies between the leading and trailing sample point. Once this has occurred, the envelope deriver circuitry is switched out of the RF signal path and the receiver now tracks the PTR of the hard limited pulse. If no adjacent pair of accumulators pass the fine settle threshold test, another set of fine settle samples is added to the existing accumulations and the detection operation is run again. The process is repeated until the PTR is found.
The fine settle thresholds are chosen to keep the probability of making an incorrect decision on the PTR to an acceptable level while maximizing the probability of making a correct decision in as short a time period as possible. A major factor which affects the PTR determination is a type of pulse distortion called Envelope Cycle Discrepancy (ECD). ECD is a differential phase delay to group delay distortion which manifests itself as a difference in arrival time between the RF pulse carrier and its envelope. Thus, although the pulse generated at a transmitter is the pulse shown in FIG. 1, the received pulse may have its carrier phase-shifted under the envelope. This alters the leading/trailing cycle peak ratio on either side of the PTR, distorting the information required for fine settle. The effect this has on a hard limited receiver is to decrease the signal level at the leading sample point for a negative ECD and to decrease the signal level at the trailing point for a positive ECD. Thus, a non-zero ECD degrades the fine settle performance by increasing both the PTR selection time and the percentage error.
It is well known that the average value of ECD of the pulses received from a particular transmitter is a function of the distance from that transmitter. It varies from a maximum of approximately a positive 2.8 microseconds within a hundred miles of a transmitter to about a negative 0.5 microseconds at a distance of 800 miles. In prior art systems using average values or emperically derived values, a fine settle threshold is chosen to meet the required error criteria (typical 1 in 10,000). This procedure is used in the design of marine Loran C receivers resulting in a receiver which meets required performance for the marine environment. While this fine settle method works well for a Loran C receiver operated in the marine environment, it does not perform well when a receiver is operated in the land mobile environment. This is due to the fact that in a marine environment, signal level and ECD variations are dominated by a dependence on distance from the receiver to transmitter. Over land, this is true only in an average sense. The proximity of the receiver to bridges, tunnels, buildings, and power lines, etc. can significantly affect received signal distortion. The total ECD observed at the receiver therefore contains a randomly varying component in addition to the distance dependent non-dynamic component. Thus, prior art systems, although adequate for marine environments, are unacceptable in a Loran C receiver which is to be used in a land mobile vehicle location system.